Battery-operated electric vehicles, especially commuter passenger automobiles, must be highly efficient in order to obtain a practical range for a given weight of batteries and to obtain desirable vehicle speeds. Even slight gains in electric efficiency are important in making such vehicles competitive with internal combustion engine vehicles. Conventionally, direct drive has been used, and the vehicle speed has been controlled by changing the armature and/or field current of the driving electric motors. This has been achieved by connecting more or fewer batteries in series to obtain a varying voltage source. More commonly, the voltage has remained constant, and the current has been increased by chopping it into greater or shorter time durations. Current-chopping generally uses silicon-controlled rectifiers or power transistors to convert dc to a variable-duty cycle dc. These require a high voltage (100 v is typical) to reduce power losses. The efficiency of such chopper circuits is about eighty-five percent maximum at half speed at their optimum voltage. An increase in efficiency is necessary for public acceptance of electric commuter vehicles.
We have devised a digitially controlled circuit that does not use chopper techniques and that exceeds in efficiency these limitations. We have achieved an efficiency higher than ninety-five percent.